1. Field of Art
This invention relates to diagnostics of spark ignition engines, and more particularly to compatible conditioning of vehicle coil primary signals, whether positively or negatively switched.
2. Description of the Prior Art
In our aforementioned copending application, the end of the dwell period of a spark ignition engine is recognized by a high voltage, spark creating swing in the low coil signal (the voltage at the ground-interrupted end of the ignition coil primary) determined from comparison with a first, fixed threshold voltage which is between the current limited voltage variations which can occur among modern electronic high voltage ignition systems and a low peak primary voltage which could be expected to be achieved with the ignition defeated, for spark analysis diagnostic procedures with the engine cranking. The system of the aforementioned application senses the beginning of the dwell period by comparing a low coil signal to a significant fraction of the actual battery voltage of the engine under test, which fraction will always exceed any low coil voltage which could exist during the dwell period. This system therefore accommodates old fashioned breaker-point ignition systems and a wide variety of different modern high voltage ignition systems, which use electronic switching in place of the old breaker points, with high battery voltage or with a poor battery during cranking.
The system of the foregoing application, and numerous other systems for conditioning the signal of the ignition coil primary, cannot be used to provide ignition coil primary signal conditioning for ignition systems which are positively switched (that is, in which the high side or battery side of the coil is switched rather than the low or ground side).